When ionizing radiation is being used, it is usually important to confine the radiation to the target volume and avoid irradiating surrounding material. This is especially important when using radiation therapy to treat tumours. So-called conformal radiotherapy seeks to deliver high radiation doses to the tumor volume and at the same time provide maximum sparing of healthy neighbouring tissues, which entails high dose gradients (steep dose falloff) outside the tumor volume. Examples of such radiotherapy techniques are Intensity Modulated Radiation Therapy (IMRT), Intra-Operative Radiotherapy (IORT) and Brachytherapy.
Commercially available external beam radiation therapy machines which can deliver an accurate and precise radiation dose to a well-defined volume in space include, for example, linear accelerator (LINAC) based machines which have been adapted to use Intensity Modulated Radiation Therapy (IMT). Commercial radiosurgery and/or radiotherapy systems presently are marketed under the brand names Trilogy® (Varian Medical Systems, Inc.), Axess® (Elekta, Inc.), X-Knife® (Radionics, Inc.), Novalis® (BrainLAB, Inc.), CyberKnife® (Accuray, Inc.) and Tomotherapy, and may use CT or MRI imaging in order to conform the radiation three-dimensionally to the target volume and minimize irradiation of surrounding healthy tissue.
The precision of delivery is affected during treatment by the inherent limitation of the imaging procedure used to delineate tumour boundary and the physician's skill in defining gross tumour volume; the latter varying from one physician to another. The dose delivery accuracy and position accuracy may also be affected by the accuracy of the dose calculation algorithm, the daily patient setup errors and mechanical positioning tolerances of the treatment machine. These effects individually or combined, will contribute to reduced accuracy and precision in the delivery of the radiation.
Further inaccuracy may be caused by movement of the target, i.e., the tumour, before and/or during therapy. The lung or breast may move significant distances because of respiration and cardiac functions. The prostate may move, as a result not only of respiration but also, for example, normal functioning of the rectum and bladder. Although feasible, it would be undesirable for the physician to compensate by increasing the irradiated volume to ensure that the entire tumour is treated, because that would likely result in increased damage to neighbouring healthy tissue.
It has been proposed to improve accuracy by tracking movement of the target volume during and/or between radiation therapy sessions. According to United States published patent application number 2006/0093089, the entire contents of both of which are incorporated herein by reference, imaging techniques, such as X-ray, CT, MRI and ultrasound, which are used to align the target volume with fiducial marks upon the patient and with the isocentre of the radiation beam equipment, are not particularly suitable for monitoring movement of the target volume caused by bodily functions. US 2006/0093089 discloses, instead, inserting one or more leadless (AC magnetic) markers into the target volume and using a panel of AC magnetic sources and sensors to determine the location of each marker in a reference frame external to the patient.
It is not enough, however, merely to ensure that the radiation is applied to the right location. It is equally important to ensure that the amount of radiation, i.e., the dose, received at a particular location is correct, according to the treatment plan. Usually, during a typical radiation therapy session, the radiation dose actually received at specified locations is measured by means of dosimeter detectors positioned in or near the target volume. The radiation level measured by a particular dosimeter detector may be affected by its proximity to an interface between two heterogeneous media and/or its proximity to a large dose gradient where dose measurements are difficult to perform accurately. For example, one might wish to irradiate the whole of the prostate gland without exposing the neighbouring rectum or urethra to damaging radiation. This imposes a need for a large transition in the radiation level over a relatively short distance. Even a slight body movement might result in the boundary of the prostate gland shifting and the rectum or urethra being subjected to an unacceptable level of radiation. Consequently, the accurate determination of the position of the detector at the time a particular radiation dose is being given is very important.
Usually, the radiation detectors are carefully positioned at the desired measurement locations before the session commences, perhaps by means of one of the above-mentioned imaging techniques and with reference to fiducial marks, for example gold markers or tattoos upon the patient, or fixed body parts. U.S. Pat. No. 6,614,025 (Thomson), commonly owned with the present invention, discloses a dosimeter having several radiation detectors referenced to a radio-opaque marker which facilitates the use of such imaging to determine the position of each radiation detector relative to the fiducial marks during the pre-treatment procedure. Nevertheless, the procedure is quite complex and provides the position at only one point in time. If the above-described movements caused by bodily functions displace a radiation detector during the actual radiation session, the dose read by the detector, when retrieved, may not be the dose actually received at the prescribed position.
The above-mentioned imaging techniques are not generally suitable for monitoring the location of a radiation detector during a radiation session. More particularly, to monitor the position of the radiation detectors while taking account of organ movement would require repeated or even continuous imaging, since a single image would not be sufficient to correlate the organ movement and the position of the detector(s). Also, it is not desirable to expose the patient to ionizing radiation in addition to that to which the patient must be exposed during therapy. Nor is it usually convenient to use an X-ray machine or other imaging machine at the same time as the radiation therapy machine.
When Brachytherapy is used instead of external beam radiation therapy, especially to treat prostate cancer, changes in the shape and size of the target volume may occur between the volume study being carried out and the radioactive isotope seeds actually being implanted. Changes also may occur during the actual implantation procedure as a result of edema and variable prostate texture causing movement in response to needle insertion and seed migration after insertion. It is particularly important, therefore, to ensure that the dose delivered to the prostate is measured accurately. As before, real-time imaging to track movement of the radiation detector(s) is not practical.